1. Technical Field
The present invention relates generally to power management in computer systems. More particularly, the present invention relates to power management in video subsystems of both portable and non-portable computers.
2. Background Art
Modern video subsystems have ever-increasing demands placed upon them. Pixel frequencies continue to increase, as well as the number of functions a video subsystem performs. As pixel frequencies and functionality increase, power requirements and power dissipation also increase. However, design changes to accommodate the increase in pixel frequencies and functionality of video subsystems have not optimized power savings and have left a lingering concern over wasted power. Although this concern exists with respect to non-portable computers, it is more acute in the portable computer market where such computers run off batteries. Extension of battery life has become a major customer concern.
Computer graphics systems exist in many forms. An exemplary system is workstation 10 depicted in FIG. 1. Connected by system bus 12 are: CPU 14; read-only-memory (ROM) 16; random access memory 18; disk drive 20; a user interface 22, which could be a keyboard and/or a mouse; and video subsystem 24. A video subsystem is also known as a display adapter. A display device 26 is connected to the video subsystem 24. Such a graphics system is known in the art and details regarding its operation are therefore not necessary.
Video subsystem 24 is depicted in expanded block diagram form in FIG. 2 and comprises a graphics controller 28, a dual port video random access memory (VRAM) 30 including a RAM portion 31 and a serial access memory (SAM) portion 32, and a serializer palette digital to analog converter (SPDAC) 34. Graphics controller 28 controls data transfers from RAM portion 31 of VRAM 30 to SAM portion 32 over line 33 and VRAM update port 35. Serial data is sent from RAM 31 to SAM 32 and out SAM port 36 to SPDAC 34 over line 38. Line 40 represents a programming interface to the SPDAC 34, which is also controlled by controller 28. The basic operation of such a video subsystem is well known in the art.
It is video subsystem 24, and SPDAC 34 in particular, where power dissipation is ripe for reduction. Power dissipation in a SPDAC chip, which is typically implemented with CMOS technology, includes both AC and DC components. As is known in the art, AC power dissipation is directly proportional to operating frequency. Thus, as operating frequency increases, so does power dissipation.
As is known in the art, power consumption in CMOS technology is approximated by the combination of standing power (i.e., when frequency is 0), and the product of a constant of proportionality, circuit capacitance, operating voltage squared, and operating frequency. Several of these power consumption components can be attacked to reduce power consumption. For example, operating voltage could be reduced, as in a move from 5 volt to 3 volt technology. Capacitance could also be reduced through better design or switching off unused circuits. In addition, standing power could be reduced through better design or switching off of the CMOS circuitry when not in use. Design changes involve costly replacement of hardware. Thus, while improved design may be practical in the future, the problem of power dissipation must be solved based on existing systems. The same is true with a move in operating voltage.
Prior art solutions to the problem of excess power dissipation in video subsystems include static power management. This type of solution only gives power savings when the video subsystem is inactive. For example, many SPDACs include a STANDBY-type mode of operation where analog power is shut-down, but the pixel clock remains running so that data can be input at all times. The clock remaining on wastes digital power needlessly. Some SPDACs also include a SLEEP-type mode of operation where both analog and digital power shut-down take place. However, the power savings are realized only during nonuse. Static power management techniques simply do not address power savings during normal video subsystem operation.
Thus, a need exists for practical power management of existing video subsystems, without replacing already existing hardware and which provides power savings during normal operation.